Method for producing martensitic stainless steel tubes

ABSTRACT

Martensitic stainless steel tubes are produced by optimizing the chemical composition of steel for billets (C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00-14.00%, N: 0.01-0.05%, P: not more than 0.020% and S: not more than 0.010%) and the relation between the soaking temperature for pierced-and-rolled tubing material and the section area reduction rate in final rolling. By this method, delayed fracture cracking in impact-receiving portions of tubes in martensitic stainless steel s such as 13% Cr tubes can be inhibited and the generation of inner-surface flaws can be prevented.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing martensitic stainless steel tubes and pipe(hereinafter, referred to as simply “tube(s)”) for use in oil wells and so forth by which cracking due to delayed fracture can be suppressed and inner-surface flaw generation can be prevented.

2. Description of the Related Art

Martensitic stainless steel tubes made of API standard 13% Cr containing about 13 mass % of chromium (Cr), for instance, are required to have yield strength of not less than 80 ksi (552 MPa) and to be provided with reasonable hot workability, so that they generally contain as much as about 0.2 mass % of C (carbon). Owing to such high Cr and C contents, the tubes as hot worked have a martensitic microstructure with a high hardness, so that they are low in toughness.

Therefore, when tubes are produced according to the conventional chemical compositions of steel and production method, affected portions thereof subjected to processing by an impact load or static load (hereinafter sometimes referred to as “impact-receiving portions”) during the period from tube-making stage to heat treatment may be cracked due to delayed fracture in some instances. For that reason, in transporting and storing steel tubes, it is necessary to take measures such that the stacking height for steel tubes is restricted and, further, reduce the stand-by time between the tube-making stage and heat treatment.

The restraints as to steel tube transportation and storage as mentioned above make it necessary to secure a large storage space in view of the restricted stacking height and, for carrying out heat treatment within a limited period of time following the tube-making process, it becomes necessary to adjust the steps from the tube-making process to heat treatment; thus, a number of difficulties arise throughout the whole tube manufacturing process.

Japanese Patent Application Publication No. 2004-43935 discloses martensitic stainless steel seamless tubes and a method of production thereof while stipulating the effective dissolved carbon content, effective dissolved nitrogen content, Cr equivalent and S content so that delayed fracture may hardly occur in impact-receiving portions and the generation of inner-surface flaws may be suppressed. However, even the technology disclosed in the publication cited above still requires that the time from the tube-making process to heat treatment be reduced. In particular, the problem of seasoning cracking, which occurs when the period from the hot tube-making process to heat treatment exceeds one week, has not yet been solved.

SUMMARY OF THE INVENTION

The present invention, which has been made in view of the problems discussed above, has for its object to provide a method for producing martensitic stainless steel tubes by which seasoning cracking due to delayed fracture is suppressed even when martensitic stainless steel tubes having a C content of about 0.2 mass % are left lying for over one week after hot tube-making until heat treatment and by which the generation of inner-surface flaws can also be prevented.

To solve the problems mentioned above, the present inventor made investigations to find out appropriate ranges of chemical compositions of billets for inhibiting impact-receiving portions from cracking due to delayed fracture and preventing the generation of inner-surface flaws as well as to find out proper temperature conditions for soaking tubing materials as pierced-and-rolled and a proper condition of section area reduction rate at a final rolling stage and, as a result, made the following findings (a) to (c). Based on such findings, the present invention has been completed.

(a) For preventing delayed fracture in impact-receiving portions by virtue of solid solution strengthening of martensitic stainless steel tubes after tube-making, it is necessary for a C content to be not more than 0.22 mass % and for a N content to be not more than 0.05 mass. For suppressing the generation of inner-surface flaws due to δ ferrite after tube-making, it is necessary that the C content is set to not less than 0.15 mass % and, for improving the hot workability to thereby inhibit the generation of inner-surface flaws, it is necessary that the content of N, which is an austenite stabilizing element, is set to not less than 0.01 mass %. A Mn content is required to be 0.10-1.00 mass % so as to secure steel strength, deoxidizing effects and hot workability.

(b) Further, when the steel contains at least one element selected from a group consisting of not more than 0.200% of V, not more than 0.200% of Nb, not more than 0.200% of Ti and not more than 0.0100% of B, each in mass %, a further effect of inhibiting delayed fracture in impact-receiving portions is exhibited, which is preferable.

(c) In order to inhibit cracking due to delayed fracture in impact-receiving portions and to prevent the generation of inner-surface flaws, it is necessary to adjust the reheating temperature T (° C.) in soaking tubing materials as obtained by piecing-and- rolling of billets having such chemical compositions as given under (a) and (b) and the section area reduction rate R (%) at a final rolling stage so that the relation represented by the inequality given below is satisfied. T>44.4×ln(R)+821  (1)

The gist of the present invention, which has been completed based on the findings mentioned above, resides in processes for producing martensitic stainless steel tubes as given below under (1) to (4).

(1) A method for producing martensitic stainless steel tubes, comprising the steps of: using a billet made of a steel containing, in mass %, C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00-14.00%, N: 0.01-0.05%, P: not more than 0.020% and S: not more than 0.010%, the balance being Fe and impurities; subjecting said billet to a piecing-and-rolling process, followed by an elongation- rolling further, thus resulting in yielding a tubing material; and soaking the resulting tubing material and subjecting the same to a final rolling process under such conditions that the reheating temperature T (° C.) in soaking prior to the final rolling process and the section area reduction rate R (%) in the final rolling satisfy the relation represented by the inequality (1) given below: T>44.4×ln(R)+821  (1) (2) In the method for producing martensitic stainless steel tubes as set forth above under (1), said billet may be made of the steel further containing, in lieu of part of Fe, at least one of V: not more than 0.200%, Nb: not more than 0.200%, Ti: not more than 0.200% and B: not more than 0.0100%, each in mass %. (3) In the method for producing martensitic stainless steel tubes as set forth above under (1) or (2), said billet may be made of the steel further containing, in lieu of part of Fe, at least one of Ni: not more than 0.5% and Cu: not more than 0.25%, each in mass %. (4) In the method for producing martensitic stainless steel tubes as set forth above under any of (1) to (3), said billet may be made of the steel further containing, in lieu of part of Fe, at least one of Al: not more than 0.1% and Ca: not more than 0.0050%, each in mass %.

The hot working method for producing seamless tubes comprises the steps of: piercing-and-rolling wherein a hole or bore is made in the central part of a solid billet; elongation-rolling wherein mainly wall thickness reduction of the tubing material obtained by the piercing-rolling process; and sizing-rolling wherein the outside diameter of the tubing material is reduced to a desired finish size.

FIG. 1 illustrates an example of the production process according to the Mannesmann tube making process for producing seamless tubes in a hot working condition. In this tube-making process, a solid billet 1 heated to a predetermined temperature is fed to a piercer rolling mill 3 for piercing-and-rolling rolling thereon to give a tubing material 2. Then, the tubing material 2 is fed to a subsequent mandrel mill (elongation-rolling mill) 4 for elongation-rolling. On the occasion of elongation-rolling at the mandrel mill 4, the tubing material 2 is cooled concurrently with elongation-rolling by means of a mandrel bar 4 b inserted in the tubing material and by means of rolling rolls 4 r, the rolling rolls controlling and adjusting the outside diameter of the tubing material. Accordingly, the tubing material 2 is then fed to a reheating furnace 5 and soaked at a predetermined reheating temperature. Thereafter, the tubing material 2 is fed to a stretch reducer 6 and undergoes final rolling, including sizing, shape correction and polishing, by means of rolling rolls 6 r.

The term “final rolling (stage)” as used herein means rolling to a specified diameter by means of a stretch reducer and/or a sizer, for instance.

The “section area reduction rate R (%) in final rolling” means the value calculated according to the equation (2) given below. R(%)={(Sin−Sout)/Sin}×100  (2)

Here, Sin represents a cross-sectional area of tube before final rolling and Sout is a cross-sectional area of tube after final rolling.

In the following description, the phrase “in mass %” for expressing each content of chemical compositions is written simply as “%”.

According to the tube-making process of the invention, by virtue of optimizing chemical compositions of the steel for billets and the relation between the soaking temperature for the tubing material as pierced-and-rolled and the section area reduction rate in final rolling, it becomes possible to inhibit cracking due to delayed fracture in impact-receiving portions of tubes made of martensitic stainless steel such as 13% Cr and, at the same time, prevent the generation of inner-surface flaws therein. Therefore, the method of the present invention is useful as the one by which martensitic stainless steel tubes for use in oil wells, among others, can be produced without confronting with any restraint in transporting and storing steel tubes and in the processing steps from tube making to heat treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating an example of the production process according to the Mannesmann tube-making process of seamless steel tubes in a hot working condition.

FIG. 2 is a drawing showing the influences of the section area reduction rate and the reheating temperature on the evaluation of steel tubes in terms of inner-surface flaws and cracking due to delayed fracture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described hereinabove, the present invention relates to a method for producing martensitic stainless steel tubes which comprises the steps of: using a billet made of a steel containing C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00-14.00%, N: 0.01-0.05%, P: not more than 0.020% and S: not more than 0.010%, the balance being Fe and impurities; subjecting said billet to piercing-and-rolling, followed by further elongation-rolling; and soaking the resultant tubing material and subjecting the same to final rolling under such conditions that the reheating temperature T (° C.) in soaking prior to final rolling and the section area reduction rate R (%) in final rolling satisfy the relation represented by the inequality (1) given hereinabove. In the following, the method of the invention is described in more detail.

(1) Chemical Compositions of Steel Tubes

The reasons why the chemical compositions of the martensitic stainless steel tubes according to the invention have been defined as described hereinabove and the preferred content ranges for the respective elements are now described.

C: 0.15-0.22%

C is an element which brings about, similarly to N, solid solution strengthening of the steel tube just after tube making, namely, for the tube formed by hot rolling and not subjected to heat treatment. For inhibiting delayed fracture in impact-receiving portions by virtue of solid solution strengthening, it is necessary for the C content to be not more than 0.22%. However, an excessive lowering of C content makes it impossible to maintain a proper level of strength after heat treatment. Further, since C is an austenite-forming element, an excessive reduction in C content leads to δ ferrite formation, possibly resulting in the generation of inner-surface seam defects after tube making. For the reasons mentioned above, it is necessary that the C content is set to not less than 0.15%. A more preferred C content range is 0.18% to 0.21%.

Si: 0.1-1.0%

Si is an element producing a deoxidizing effect for steel. For obtaining such effect, it is necessary for the Si content is set to not less than 0.1%. At high Si content levels exceeding 1.0%, however, toughness is deteriorated. For such reasons, the Si content range of 0.1-1.0% is considered appropriate. When priority is given to securing of toughness, the Si content is preferably set to not more than 0.75%. Amore preferred Si content range is 0.20-0.35%.

Mn: 0.10-1.00%

Mn is an element having a strength-improving effect for steel and, like Si, having a deoxidizing effect. Further, it immobilizes S in steel as MnS, producing a hot workability-improving effect. For obtaining those effects, it is necessary for the Mn content to be not less than 0.10%. However, at high content levels exceeding 1.00%, toughness is deteriorated. For the above reasons, the Mn content range of 0.10-1.00% is considered appropriate.

Cr: 12.00-14.00%

Cr constitutes a basic component having a corrosion resistance-improving effect for steel. In particular, when the Cr content is not less than 12.00%, pitting resistance and crevice corrosion resistance are improved and corrosion resistance in a CO₂ atmosphere is markedly improved. On the other hand, at high Cr content levels exceeding 14.00%, δ ferrite is readily formed during working at high temperatures since Cr is a ferrite-forming element; as a result, the hot workability is impaired. In addition, the addition of an excessive amount of Cr results in an increase in production cost. For the above reasons, the Cr content range of 12.00-14.00% is considered appropriate. A more preferred Cr content range is 12.40-13.10%.

N: 0.01-0.05%

N is an austenite-stabilizing element and is effective in improving the hot workability of steel and preventing the generation of inner-surface flaws. For obtaining those effects, it is necessary for the N content to be not less than 0.01%. High N content levels exceeding 0.05%, however, induce the delayed fracture of impact-receiving portions. Therefore, the N content range of 0.01-0.05% is considered appropriate. The N content is more preferably 0.02-0.035%.

P: not more than 0.020%

P is an impurity element in steel. At high P content levels, toughness of products after heat treatment lowers and, therefore, the upper limit to the content thereof is set to 0.020%. It is preferred that the P content is as low as possible.

S: not more than 0.010%

S is an impurity element reducing the hot workability of steel when the content thereof is high and, therefore, the allowable upper limit to the content thereof is set to 0.010%. It is preferred that the S content is as low as possible, more preferably not more than 0.003%.

One or more of V, Ti, Nb and B:

These elements may or may not be contained in steel. When contained, these elements produce an effect of inhibiting delayed fracture in impact-receiving portions. For obtaining such effect, it is preferred that one or more of these elements be contained in the steel. As for the contents thereof, a content of not less than 0.005% is preferred for each of V, Ti and Nb, and a content of not less than 0.0005% is preferred for B.

On the other hand, when the content of these elements is excessive, the increased hardness resulting from nitride formation after heat treatment causes deterioration in corrosion resistance and/or decreases in toughness and induces fluctuations in strength as well. Therefore, for V, Ti and Nb, the content of each of them is preferably not more than 0.200% and, for B, the content thereof is preferably set to not more than 0.0100%.

One or more of Ni and Cu:

These elements may or may not be contained in steel. When one or more of them are contained, they produce an effect of improving the hot workability and corrosion resistance of the steel.

Ni: When contained in the steel, Ni produces an effect of improving the hot workability of steel since Ni is an austenite-stabilizing element. For obtaining that effect, the Ni content is preferably not less than 0.001%. On the other hand, at excessively high content levels, the sulfide stress corrosion cracking resistance decreases. Therefore, the content thereof is preferably set to not more than 0.5%.

Cu: Cu is a corrosion resistance-improving element and is an austenite-stabilizing element and, therefore, when contained in steel, it produces an effect of improving the hot workability of steel. For obtaining such effect, the content thereof is preferably not less than 0.001%. On the other hand, Cu is a low-melting-point metal and, when the content thereof is excessive, the hot workability is decreased on the contrary. Therefore, the content thereof is preferably set to not more than 0.25%.

one or more of Al and Ca:

These elements may or may not be contained in steel. When contained, they are effective in inhibiting the generation of outside-surface seam defects and/or preventing the hot workability from decreasing.

Al: When Al is contained in steel, it effectively acts as a steel deoxidizing agent and also has an effect of inhibiting the generation of outside-surface seam defects. For obtaining such effects, the content thereof is preferably not less than 0.001%. On the other hand, an excessive content thereof leads to a decrease in cleanliness of steel or non-metallic inclusion levels and further causes an immersion nozzle clogging in continuous casting. Therefore, the Al content is preferably set to not more than 0.1%.

Ca: When contained in steel, Ca unites with S in steel and produces an effect of preventing the hot workability from decreasing due to grain boundary segregation of S. For obtaining those effects, the content thereof is preferably set to not. less than 0.0001%. On the other hand, an excessive content of Ca causes the generation of macro-streak flaws and, therefore, the content thereof is preferably set to not more than 0.0050%.

(2) Relation Between the Reheating Temperature in Soaking and the Section Area Reduction Rate in Final Rolling

Billets each having a chemical composition of steel comprising the components within the respective appropriate ranges mentioned above and an outside diameter of 190 mm were subjected to piercing-rolling, followed by elongation-rolling. The thus-obtained tubing material were soaked at various reheating temperatures T (° C.) and subjected to final rolling while the section area reduction rate R (%) was varied. The steel tubes thus produced were subjected to fundamental testing for examining the occurrence of cracking due to delayed fracture and the generation of inner-surface flaws. In part of the fundamental testing, the testing was carried out also using billets in which the N content and/or Mn content in steel deviate from the respective appropriate ranges.

In piercing-rolling, a two-roll inclined rolling mill having conical rolls was used, and the angle of inclination was 12-15° and the crossing angle was 10°.

The chemical compositions of steel for billets, steel tube production conditions and test results are summarized in Table 1. TABLE 1 Production conditions Results evaluation Extent of Reheating Inner Steel composition reduction in temperature T surface Crack Overall Test No. N(%) Mn(%) area R (%) (° C.) flaw rating rating rating A1 0.030 0.250 65.0 980 ◯ X X A2 0.030 0.250 65.0 990 ◯ X X A3 0.030 0.250 65.0 1000 ◯ Δ Δ A4 0.030 0.250 65.0 1010 ◯ ◯ ◯ AS 0.030 0.250 65.0 1020 ◯ ◯ ◯ A6 0.030 0.250 55.5 970 ◯ X X A7 0.030 0.250 55.5 980 ◯ X X A8 0.030 0.250 55.5 990 ◯ Δ Δ A9 0.030 0.250 55.5 1000 ◯ ◯ ◯ A10 0.030 0.250 55.5 1010 ◯ ◯ ◯ A11 0.030 0.250 43.4 960 ◯ X X A12 0.030 0.250 43.4 970 ◯ X X A13 0.030 0.250 43.4 980 ◯ Δ Δ A14 0.030 0.250 43.4 990 ◯ ◯ ◯ A15 0.030 0.250 43.4 1000 ◯ ◯ ◯ A16 0.030 0.250 30.9 950 ◯ X X A17 0.030 0.250 30.9 960 ◯ X X A18 0.030 0.250 30.9 970 ◯ Δ Δ A19 0.030 0.250 30.9 980 ◯ ◯ ◯ A20 0.030 0.250 30.9 990 ◯ ◯ ◯ A21 0.030 0.250 21.3 930 ◯ X X A22 0.030 0.250 21.3 940 ◯ X X A23 0.030 0.250 21.3 950 ◯ Δ Δ A24 0.030 0.250 21.3 960 ◯ ◯ ◯ A25 0.030 0.250 21.3 970 ◯ ◯ ◯ A26 0.030 0.250 5.3 870 ◯ X X A27 0.030 0.250 5.3 880 ◯ X X A28 0.030 0.250 5.3 890 ◯ Δ Δ A29 0.030 0.250 5.3 900 ◯ ◯ ◯ A30 0.030 0.250 5.3 910 ◯ ◯ ◯ A31 *0.009 0.250 55.5 1000 X ◯ X A32 0.010 0.250 55.5 1000 ◯ ◯ ◯ A33 0.030 0.250 55.5 1000 ◯ ◯ ◯ A34 0.050 0.250 55.5 1000 ◯ ◯ ◯ A35 *0.060 0.250 55.5 1000 ◯ Δ Δ A36 0.030 *0.080 55.5 1000 X ◯ X A37 0.030 0.100 55.5 1000 ◯ ◯ ◯ A38 0.030 0.500 55.5 1000 ◯ ◯ ◯ A39 0.030 1.000 55.5 1000 ◯ ◯ ◯ A40 0.030 *1.050 55.5 1000 ◯ Δ Δ Note: The mark * indicates that the relevant content is outside the range speficied by the present invention.

In Table 1, the section area reduction rate R (%) means the section area reduction rate in final rolling and each value shown was calculated according to the formula (2) given below. R(%)={(Sin−Sout)/Sin}×100  (2)

In the above formula, Sin represents the cross-sectional area of the steel tube before final rolling and Sout represents the cross-sectional area of the steel tube after final rolling.

The test results were evaluated according to the evaluation criteria given below, and the evaluation results are shown in the column under the heading “Results evaluation” in the above Table.

Inner-surface Flaw Rating:

Not less than 50 lengths of steel tubes having a length of 10-12.5 m were produced and examined for the generation of inner-surface flaws by visual inspection and ultrasonic testing (UST). The mark ∘ was given when the inner surface flaw incidence rate was not more than 3% and, when the inner surface flaw incidence rate exceeded 3%, the mark x was given.

Crack Rating:

Drop weight test specimens, 250 mm in length, were collected from the steel tubes after production, a block weight having a tip radius of curvature of 90 mm and a mass of 150 kg was dropped on the specimens from a height of 0.2 m (i.e. an impact load of 294 J was given thereto) to cause deformation, and the presence or absence of cracks was checked by visual inspection and UST. Then, when there was no crack generation after 1 week, the mark ∘ was given and, when no crack generation was observed after 3 days, the mark Δ was given. When crack generation was observed after 3 days, the mark x was given.

Overall Rating:

Out of the above-mentioned inner-surface flaw evaluation and crack evaluation results, the poorest rating among all constitutes the overall evaluation result.

Further, out of the test results shown above in Table 1, the results of those tests in which the N and Mn contents in steel were within the respective appropriate ranges specified in claim 1 were used to collate the influences of the section area reduction rate and reheating temperature on the overall rating in terms of the inner-surface flaw and delayed fracture cracking.

FIG. 2 is a graphic representation of the influences of the section area reduction rate and reheating temperature on the overall rating in terms of the steel tube inner-surface flaw and delayed fracture cracking development ratings.

Based on the results shown in Table 1 and FIG. 2, a borderline between the cases where the overall rating was ∘ and the cases where the overall rating was Δ or x was determined in terms of a relation between the section area reduction rate R (%) in final rolling and the reheating temperature T (° C.) in soaking. As a result, the following formula (3) was obtained. T=44.4×Ln(R)+821  (3)

From the relation represented by the above formula (3), it was revealed that for inhibiting the delayed fracture cracking in impact-receiving portions and preventing the generation of inner-surface flaws, it is necessary to adjust the reheating temperature T (° C.) in soaking the tubing material obtained by piercing-and-rolling and the section area reduction rate R (%) in final rolling so that they satisfy the relation represented by the above equation (1).

In test numbers A31 and A35 in which the N content in steel was outside the appropriate range and in test numbers A36 and A40 in which the Mn content was outside the appropriate range, inner-surface flaws or cracking generated and the overall rating was Δ or x.

EXAMPLES

For confirming the effects of the martensitic stainless steel tube production method according to the invention, a steel tube production test was carried out as described below, and the results were evaluated.

In the same manner as the fundamental testing described above, billets varied in chemical composition of steel and having an outside diameter of 190 mm were subjected to piercing-and-rolling, soaking of thus-produced tubing material at various reheating temperatures T (° C.) and subjecting the same to final rolling while varying the section area reduction rate R (%), and the steel tubes after tube making were examined for the occurrence of delayed fracture cracking and for the generation of inner-surface flaws.

In piercing-and-rolling, a two-roll inclined rolling mill having conical rolls, and the angle of inclination was 12-15° and the crossing angle was 10°.

The chemical compositions of steel for billets, steel tube production conditions and test results are summarized in Table 2 and Table 3. TABLE 2 Chemical composition (in mass %, the balance being Fe and impurities) Test No. C Si P S Cr Ni Cu V Al Nb Ti B Ca Mn N B1 0.20 0.24 0.014 0.0005 12.46 — — — 0.002 — — — — 0.460 0.010 B2 0.20 0.23 0.013 0.0008 12.44 — — — 0.003 — — — — 0.470 0.050 B3 0.19 0.23 0.014 0.0005 12.42 — — — 0.003 — — — — 0.100 0.032 B4 0.21 0.29 0.013 0.0006 12.57 — — — 0.003 — — — — 1.000 0.032 B5 0.22 0.25 0.014 0.0006 12.63 — — — 0.003 — — — — 0.470 *0.008 B6 0.19 0.25 0.014 0.0005 12.43 — — — 0.004 — — — — 0.470 *0.053 B7 0.20 0.23 0.013 0.0006 12.44 — — — 0.002 — — — — *0.070 0.031 B8 0.18 0.23 0.014 0.0006 12.52 — — — 0.002 — — — — *1.050 0.032 B9 0.20 0.24 0.013 0.0005 12.48 — — — 0.002 — 0.005 — — 0.470 0.030 B10 0.20 0.26 0.011 0.0007 12.43 — — — 0.003 — 0.001 0.0004 — 0.450 0.029 B11 0.19 0.24 0.013 0.0006 12.47 0.110 0.010 — 0.003 0.001 — — 0.0007 0.480 0.029 B12 0.19 0.24 0.013 0.0006 12.47 0.110 0.010 — 0.003 0.001 — — 0.0007 0.480 0.029 B13 0.19 0.24 0.013 0.0006 12.47 0.110 0.010 — 0.003 0.001 — — 0.0007 0.480 0.029 B14 0.19 0.24 0.013 0.0006 12.47 0.110 0.010 — 0.003 0.001 — — 0.0007 0.480 0.029 B15 0.19 0.24 0.013 0.0006 12.47 0.110 0.010 — 0.003 0.001 — — 0.0007 0.480 0.029 B16 0.21 0.23 0.013 0.0005 12.55 0.130 0.010 — 0.002 0.001 0.002 — 0.0008 0.480 0.030 B17 0.21 0.23 0.013 0.0005 12.55 0.130 0.010 — 0.002 0.001 0.002 — 0.0008 0.480 0.030 B18 0.21 0.23 0.013 0.0005 12.55 0.130 0.010 — 0.002 0.001 0.002 — 0.0008 0.480 0.030 B19 0.21 0.23 0.013 0.0005 12.55 0.130 0.010 — 0.002 0.001 0.002 — 0.0008 0.480 0.030 B20 0.21 0.23 0.013 0.0005 12.55 0.130 0.010 — 0.002 0.001 0.002 — 0.0008 0.480 0.030 B21 0.18 0.23 0.013 0.0005 12.52 0.110 0.010 0.070 0.002 — — — 0.0006 0.570 0.030 B22 0.18 0.23 0.013 0.0005 12.52 0.110 0.010 0.070 0.002 — — — 0.0006 0.570 0.030 B23 0.18 0.23 0.013 0.0005 12.52 0.110 0.010 0.070 0.002 — — — 0.0006 0.570 0.030 B24 0.18 0.23 0.013 0.0005 12.52 0.110 0.010 0.070 0.002 — — — 0.0006 0.570 0.030 B25 0.18 0.23 0.013 0.0005 12.52 0.110 0.010 0.070 0.002 — — — 0.0006 0.570 0.030 B26 0.19 0.24 0.014 0.0006 12.50 0.130 0.020 — 0.001 0.002 0.002 — 0.0003 0.460 0.029 B27 0.19 0.24 0.014 0.0006 12.50 0.130 0.020 — 0.001 0.002 0.002 — 0.0003 0.460 0.029 B28 0.19 0.24 0.014 0.0006 12.50 0.130 0.020 — 0.001 0.002 0.002 — 0.0003 0.460 0.029 B29 0.19 0.24 0.014 0.0006 12.50 0.130 0.020 — 0.001 0.002 0.002 — 0.0003 0.460 0.029 B30 0.19 0.24 0.014 0.0006 12.50 0.130 0.020 — 0.001 0.002 0.002 — 0.0003 0.460 0.029 B31 0.20 0.24 0.015 0.0009 12.59 0.080 0.020 — 0.002 0.002 0.001 — 0.0009 0.480 0.026 B32 0.20 0.24 0.015 0.0009 12.59 0.080 0.020 — 0.002 0.002 0.001 — 0.0009 0.480 0.026 B33 0.20 0.24 0.015 0.0009 12.59 0.080 0.020 — 0.002 0.002 0.001 — 0.0009 0.480 0.026 B34 0.20 0.24 0.015 0.0009 12.59 0.080 0.020 — 0.002 0.002 0.001 — 0.0009 0.480 0.026 B35 0.20 0.24 0.015 0.0009 12.59 0.080 0.020 — 0.002 0.002 0.001 — 0.0009 0.480 0.026 B36 0.19 0.23 0.012 0.0005 12.50 0.010 0.010 0.070 0.002 — 0.002 — 0.0004 0.590 0.031 B37 0.19 0.23 0.012 0.0005 12.50 0.010 0.010 0.070 0.002 — 0.002 — 0.0004 0.590 0.031 B38 0.19 0.23 0.012 0.0005 12.50 0.010 0.010 0.070 0.002 — 0.002 — 0.0004 0.590 0.031 B39 0.19 0.23 0.012 0.0005 12.50 0.010 0.010 0.070 0.002 — 0.002 — 0.0004 0.590 0.031 B40 0.19 0.23 0.012 0.0005 12.50 0.010 0.010 0.070 0.002 — 0.002 — 0.0004 0.590 0.031 Note: The mark * indicates that the relevant content is outside the range specified by the present invention.

TABLE 3 Production conditions Results evaluation Section area Reheating Value of Inner Test reduction temperature right side of Crack surfacae Overall No. rate R (%) T (° C.) equation (1) rating flaw rating rating Distinction Remarks B1 21.3 1,000 957 ◯ ◯ ◯ Inventive example B2 21.3 1,000 957 ◯ ◯ ◯ Inventive example B3 21.3 1,000 957 ◯ ◯ ◯ Inventive example B4 21.3 1,000 957 ◯ ◯ ◯ Inventive example 85 21.3 1,000 957 ◯ X X Comparative example Inner surface flaw generation due to decreased hot workability B6 21.3 1,000 957 X ◯ X Comparative example Cracking due to decreased delayed fracture resistance B7 21.3 1,000 957 ◯ X X Comparative example Inner surface flaw generation due to decreased hot workability B8 21.3 1,000 957 X ◯ X Comparative example Cracking due to deteriorated toughness B9 21.3 1,000 957 ◯ ◯ ◯ Inventive example B10 21.3 1,000 957 ◯ ◯ ◯ Inventive example B11 65.0 *980 1,006 X ◯ X Comparative example B12 65.0 *990 1,006 X ◯ X Comparative example B13 65.0 *1000 1,006 Δ ◯ Δ Comparative example B14 65.0 1,010 1,006 ◯ ◯ ◯ Inventive example B15 65.0 1,020 1,006 ◯ ◯ ◯ Inventive example B16 55.5 *970 999 X ◯ X Comparative example B17 55.5 *980 999 X ◯ X Comparative example B18 55.5 *990 999 Δ ◯ Δ Comparative example B19 55.5 1,000 999 ◯ ◯ ◯ Inventive example B20 55.5 1,010 999 ◯ ◯ ◯ Inventive example B21 43.4 *960 988 X ◯ X Comparative example B22 43.4 *970 988 X ◯ X Comparative example B23 43.4 *980 988 Δ ◯ Δ Comparative example B24 43.4 990 988 ◯ ◯ ◯ Inventive example B25 43.4 1,000 988 ◯ ◯ ◯ Inventive example B26 30.9 *950 973 X ◯ X Comparative example B27 30.9 *960 973 X ◯ X Comparative example B28 30.9 *970 973 Δ ◯ Δ Comparative example B29 30.9 980 973 ◯ ◯ ◯ Inventive example B30 30.9 990 973 ◯ ◯ ◯ Inventive example B31 21.3 *930 957 X ◯ X Comparative example B32 21.3 *940 957 X ◯ X Comparative example B33 21.3 *950 957 Δ ◯ Δ Comparative example B34 21.3 960 957 ◯ ◯ ◯ Inventive example B35 21.3 970 957 ◯ ◯ ◯ Inventive example B36 5.3 *870 895 X ◯ X Comparative example B37 5.3 *880 895 X ◯ X Comparative example B38 5.3 *890 895 Δ ◯ Δ Comparative example B39 5.3 900 895 ◯ ◯ ◯ Inventive example B40 5.3 910 895 ◯ ◯ ◯ Inventive example Note: The mark * indicates that the relevant content is outside the range specified by the present invention.

The test results were evaluated according to the same evaluation criteria as those used in the above-mentioned fundamental testing.

Test Nos. B1-B4, B9, B10, B14, B15, B19, B20, B24, B25, B29, B30, B34, B35, B39 and B40 are Inventive Examples which respectively satisfy the chemical composition range requirements specified by the present invention. Test Nos. B5-B8 are Comparative Examples in which the N content or Mn content does not meet the content range requirement established by the invention, and Test Nos. B11-B13, B16-B18, B21-B23, B26-B28, B31-B33 and B36-B38 are Comparative Examples in which the relation between the section area reduction rate R and the reheating temperature T does not satisfy the relation defined by the formula (1) given hereinabove.

In the above-mentioned examples according to the invention, neither delayed fracture cracking after tube making nor inner-surface flaw generation occurred and, thus, steel tubes of good quality were obtained. In Test Nos. B9, B10, B14, B15, B19, B20, B24, B25, B29, B30, B34, B35, B39 and B40, in particular, which are Inventive Examples and in which the chemical compositions each contained one or more of V, Ti, Nb and B, or one or more of Ni and Cu, or one or more of Al and Ca, the delayed fracture resistance performance and hot workability, among others, were much better.

On the contrary, in Test Nos. of Comparative Example B5-B8, in which the N content or Mn content in the chemical composition of steel failed to meet the range requirement established by the invention, inner-surface flaw generation due to a decrease in hot workability, cracking due to a decrease in delayed fracture resistance and cracking due to a decrease in toughness were observed and the steel tubes obtained were poor in quality. In Test Nos. of Comparative Example B11-B13, B16-B18, B21-B23, B26-B28, B31-B33 and B36-B38, delayed fracture cracking and/or inner-surface flaws were generated respectively and the steel tubes obtained were poor in performance.

As described hereinabove, the steel tube production method of the invention can inhibit the cracking due to delayed fracture and prevent the generation of inner-surface flaws in impact-receiving portions of tubes made of martensitic stainless steel such as 13% Cr by optimizing the chemical composition of steel for billets and the relation between the soaking temperature for tubing materials as pierced-and-rolled and the section area reduction rate in final rolling. Therefore, the method of the invention can be applied widely in the field of production of martensitic stainless steel tubes for use in oil wells and so forth as a method by which high-quality steel tubes can be produced without any particular restraint in transporting and storing steel tubes and in the processing steps from tube making to heat treatment. 

1. A method for producing martensitic stainless steel tubes, comprising the steps of: using a billet having a chemical composition containing C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00-14.00%, N: 0.01-0.05%, P: not more than 0.020% and S: not more than 0.010%, each in mass %, the balance being Fe and impurities; subjecting said billet to piercing-and-rolling, followed by further elongation-rolling, thus yielding a tubing material; and soaking thus-obtained tubing material and subjecting the same to final rolling under such conditions that a reheating temperature T (° C.) in soaking prior to the final rolling and a section area reduction rate R (%) in final rolling satisfy the relation represented by an inequality formula (1) given below: T>44.4×ln(R)+821  (1).
 2. The method for producing martensitic stainless steel tubes as set forth in claim 1, wherein the chemical composition of said billet includes, in lieu of a part of Fe, one or more of V: not more than 0.200%, Nb: not more than 0.200%, Ti: not more than 0.200% and B: not more than 0.0100%, each in mass %.
 3. The method for producing martensitic stainless steel tubes as set forth in claim 1, wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Ni: not more than 0.5% and Cu: not more than 0.25%, each in mass %.
 4. The method for producing martensitic stainless steel tubes as set forth in claim 2, wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Ni: not more than 0.5% and Cu: not more than 0.25%, each in mass %.
 5. The method for producing martensitic stainless steel tubes as set forth in claim 1, wherein the chemical composition of said billet further included, in lieu of part of Fe, one or more of Al: not more than 0.1% and Ca: not more than 0.0050%, each in mass %.
 6. The method for producing martensitic stainless steel tubes as set forth in claim 2, wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Al: not more than 0.1% and Ca: not more than 0.0050%, each in mass %.
 7. The method for producing martensitic stainless steel tubes as set forth in claim 3, wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Al: not more than 0.1% and Ca: not more than 0.0050%, each in mass %.
 8. The method for producing martensitic stainless steel tubes as set forth in claim 4, wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Al: not more than 0.1% and Ca: not more than 0.0050%, each in mass %. 